Conventionally, there has been proposed a rotary atomizing coating device which delivers the coating supplied from a coating machine through a coating outlet hole provided at a hub member by using the centrifugal force of the rotation, to a coating diffusion surface which is provided at an inner periphery of a head body and configured by a curved surface with a concave shape towards the rotation axial line, and which atomizes the coating near the outer edge of the coating diffusion surface to spray towards an object to be coated (refer to Patent Literature 1).
In this device, since the coating diffusion surface is configured by a curved surface having a concave shape towards the rotation axial line, it is able to quickly supply the liquid film of the coating to the outer edge of the coating diffusion surface.
However, since the coating moving in the same radial direction has a tendency of decreasing the moving acceleration as moving towards the outer edge of the coating diffusion surface, there are cases where the liquid film of the coating becomes thicker at the coating diffusion surface configured by the curved surface with the concave shape compared to a coating diffusion surface configured by a curved surface with a convex shape which is described later. The reason for this can be understood from the fact that when the centrifugal force is indicated as F, and the angle between the rotation axial line and the direction along the curved surface with the concave shape is indicated as θ, the component force of the centrifugal force in the direction along the curved surface with the concave shape expressed by Fsin θ becomes smaller towards the outer edge of the coating diffusion surface, and in the contrary, the component force of the centrifugal force in a direction vertical to the direction along the curved surface with the concave shape expressed by Fcos θ becomes larger towards the outer edge of the coating diffusion surface.
As a result, there is a possibility that the liquid film of the coating which has become thick in the vicinity of the outer edge of the coating diffusion surface overflows from the groove formed in the vicinity of the outer edge of the coating diffusion surface, and there is a concern that the overflowed coating is sprayed while not being formed as liquid threads. Moreover, in a case where the liquid film of the coating becomes very thick, there is a possibility that it separates from the curved surface with the concave shape before reaching the vicinity of the outer edge of the coating diffusion surface, thereby causing defects in the appearance of the coating object.
Moreover, there has been proposed a rotational atomization electrostatic coating apparatus in which a coating supplied from the coating nozzle is delivered through a coating outlet hole to a coating diffusion surface which is configured by a curved surface with a convex shape towards a rotation axial line and provided at the inner periphery of a spray head, and the coating is atomized in the vicinity of the outer edge of the coating diffusion surface to be sprayed towards the coating object (refer to Patent Literature 2).
In such apparatus, since the coating diffusion surface is configured by a curved surface with a convex shape towards the rotation axial line, the liquid film of the coating becomes thinner towards the outer edge of the coating diffusion surface. Therefore, the inconvenience of the liquid film becoming thick can be avoided.
However, since there is a tendency that the acceleration of the coating becomes larger towards the outer edge of the coating diffusion surface, the moving speed of the coating becomes too fast in the vicinity of the outer edge of the coating diffusion surface that the coating does not enter the grooves formed in the vicinity of the outer edge of the coating diffusion surface, thereby being sprayed without being formed as liquid threads,